In order to record a piano performance and reproduce it mechanically; that is, by re-creating the performance on a piano by mechanical means, it is essential to measure the dynamics of the performance with a high degree of accuracy. Ideally, the method used for measuring the dynamics should yield an independent dynamic value for each note that is played. In addition, the method should not alter the "feel" of the piano in any way, and should be highly reliable. These requirements are met by employing non-contacting sensing of "near terminal hammer velocity."
With respect to the foregoing, true terminal hammer velocity is defined as the linear velocity of the hammer at the instant it collides with the string. Such true terminal hammer velocity is difficult to measure. However, it can be approximated very closely by the average hammer velocity measured over a small displacement of the hammer, just prior to the point of impact. The term "near terminal hammer velocity" is given to this approximation to true terminal hammer velocity.
The foregoing experimental result is explained by three facts:
First, an escapement within the piano action releases a hammer from its actuating mechanism a short distance before the hammer impacts the string, so that the hammer is in free flight, subject only to the influences of gravity and friction, before it hits the string. In a properly regulated grand piano, this free flight starts approximately three millimeters before impact with the string.
Second, as the hammer approaches let-off (as the escapement point is called) the mechanical advantage between key and hammer decreases gradually, making it increasingly difficult to accelerate the hammer.
Third, a human pianist is incapable of large accelerations in his arms and fingers. He achieves high velocities by accelerating a hammer throughout its stroke, or by attacking the key from a large height, which accelerates the hammer early in the stroke.
While systems have been proposed for attempting to measure the hammer velocities and thus provide a measurement of the dynamics of a piano performance, they often require radical modification of the particular piano involved; for example, a complete relocation of the various piano playing components relative to the strings in order to accommodate the necessary mechanism to make the measurements in question. Further, there is great risk in many devices in altering the "feel" of the piano when attempting to measure individually the velocity of the hammer associated with each note played.